Electrical apparatus having insulation for eliminating creepage tracking



Aug. 22, 1961 A. A. KESSEL ET AL ELECTRICAL APPARATUS HAVING INSULATION FOR ELIMINATING CREEPAGE TRACKING 5 sheets sheet 1 Filed Jan. 9, 1957 n r I I m uz m lIlIIIIl/I lnvenfors: Alvon A. Kessel Robefi S. Norman b .WJ Aug Their AHornev Aug. 22, 1961 A. A. KESSEL ET A]. 2,997,526

ELECTRICAL APPARATUS HAVING INSULATION FOR ELIMINATING CREEPAGE TRACKING Filed Jan. 9, 1957 3 Sheets-Sheet 2 Surface erosion Type 250 *yp'. ail-u e cilur Hours f0 failure 0 IO 20 3O 4O 5O 6O by weighi hydrufed Alumina lnventors: Alvqn A. Kessel Roberf 5. Norman Aug. 22, 1961 A. A. KESSEL ET AL 2,997,526

ELECTRICAL APPARATUS HAVING INSULATION FOR ELIMINATING CREEPAGE TRACKING Filed Jan. 9, 1957 3 Sheets-Sheet 3 lnven+ors2 Alvcm A. Kessd Roberf S. Norman Their AHoTney U i d see PM 2,997,526 ELECTRICAL APPARATUS HAVING INSULATION FOR ELIMINATING CREEPAGE TRACKING Alvan A. Kessel, Wakefield,'and Robert S. Norman, Wm-

throp, Mass., assignors to General Electric Company, a corporation of New York Filed Jan. 9, 1957, Sex. No. 633,356 6 Claims. (Cl. 174-137) The present invention relates to improvements in the insulation of electrical apparatus and, more particularly, to electrical devices and improved insulations therefor which are subject to contaminated conditions which promote creepage electrical discharge conditions. As is well known, certain types of electrical equipment subject to contaminating atmospheric conditions such as moisture, dust, fog and salt frequently fail due to creepage between points of different potentials on the equipment. While the insulating components of electrical equipment desirably include organic materials which are inexpensive and readily molded or otherwise fabricated, it has been a particular disadvantage that the organic constituents of such components tend to form carbonaceous deposits upon exposure to conditions which promote such creepage. These carbonaceous deposits ultimately provide paths of suificiently low resistance to occasion breakdown of the equipment. This application is a continuation-in-part of 'our copending application, Serial No. 523,445, filed July 21, 1955, now abandoned.

By way of example in the instrument transformer field, it has heretofore been a-distinctlimitation to the usefulness of dry molded instrument transformers that the molded organic insulation components thereof form lowresistance carbonaceous deposits upon exposure to creep age electrical discharges. High-voltage transformers of this dry type may includelmolded organic insulating compounds not only as the insulation between elements but as the outer protectivecasing as well, and are thus distinguished from those which are filled with oil or other insulating liquids. In outdoor installations, or others where there may be accumulations of dust, rain and other environmental contaminants,-random surface discharges or arcing known as surface creepage are promoted between elements. These discharge conditions occasion the formation of carbonaceous deposits in the insulation, ultimaely yielding low-resistance paths or tracks which destroy further utility of the apparatus. Discharges of the creepage type are to be distinguished from those caused by the establishment of an arc through or directly between two parts of the apparatus having diiferent potentials. Under arcing conditions, while the organic material adjacent to the arc is carbonized, the arc track so formed is not randomin character but forms a direct path along the line of the arc. On the other hand, tracks due to creepage are random in effect and produce a treelike path. The difference between tracking due to arcing and tracking due to creepage is further pointed out in ASTM test D495-48T in which it is stated specifically that the test directed to determining the resistance of insultating material to arcs does not in general permit conclusions to be drawn as to the resistance of the material to other types of are such as those promoted by conducting contaminates with which latter the present invention is concerned. It is further pointed out that in the creepage type of electrical failure, failure of the material can occur not only due to surface failure but to subsurface failure. In other words, even if the surface of the organic material is devoid of carbonaceous or conducting material, tracking due to surface creepage may nevertheless occur between two points of different potential beneath the surface of the material itself. It is evident from the above and it has been found that materials which are ice 2 L effective in protecting against the effects of direct arcing are not necessarily effective in protecting against creep age breakdown.

As a result of such disadvantages, organic insulating components have been avoided in the construction of electrical equipment wherein such components would be subjected to the influences of creepage electrical discharges, even though these components would otherwise have been attractive because of different considerations. Organic insulating materials which experience this limitation are Butyl rubber, epoxy resins and polyester resins.

Accordingly, it is one object of this invention to provide improved electrical apparatus having organic insulation which eliminates tracking due to creepage electrical discharge conditions.

It is another object to provide improved electrical apiparatus including certain organic compositions which have insulating characteristics and in which formation of carbonaceous deposits upon exposure to creepage type electrical discharges is eliminated.

By way of a summary account of one practice of the teachings of this invention, an improved high-voltage outdoor instrument transformer includes a magnetic time member with linked primary and secondary windings each connected to terminals, and a particular organic composition having hydrated alumina dispersed therein in proportions defined hereinafter. The organic composition with the hydrated alumina is formed about the transformer elements to electrically insulate them and to provide an outer covering which protects these elements against influences of the ambient environment. Uniquely, the creepage discharges occurring across the surface of the transformer even under the most severe contaminating conditions do not occasion tracking and breakdown, inasmuch as low-resistance carbonaceous deposits are not permitted to form. Generally, the organic materials which may be combined with the hydrated alumina in this insulating component with particular advantage include Butyl gum, and epoxy and polyester resins.

Although the features of this invention which are novel are set forth in the appended claims, greater detail of the invention in its preferred embodiments and the further objects and advantages thereof may be readily comprehended through reference to the following description taken in connection with the accompanying drawings, wherein:

FIGURE 1 provides a perspective view of a molded casing type transformer embodying our invention;

FIGURE 2 is a cross-sectional view of the transformer of FIGURE 1;

FIGURE 3 is a graphical presentation of test data obtained with formulations including Butyl rubber;

FIGURE 4 is a pictorial view of electrical test equipment utilized to procure operating life data; and

FIGURE 5 depicts an outdoor high-voltage transformer embodying our invention, portions of the transformer being cut away to reveal constructional details.

Referring to FIGURES l and 2 of the drawings, the electrical apparatus there shown is a current transformer having primary terminals 10 and 111 and secondary terminals 12 and 13. The positioning of the primary windings 14 and the secondary windings 15 will be apparent from the view of FIGURE 2. These windings are electromagnetically linked with a core 16 of magnetic material which is provided with a pair of tubular separators 17 and 18 which serve to insulate the core from the windmgs.

The transformer of FIGURES 1 and 2 includes composition 19 which encapsulates the various components as shown and thereby provides a completed physical enclosure as Well as electrical insulation. A support plate 20 facilitates mounting. As has been pointed out above,

organic insulating materials heretofore employed as such components of transformers of this type, and in other applications wherein similar conditions may occur, have been subject to the basic problem of carbonaceous res1-. -due formation and accumulation occasioned by random electrical creepage discharges which are likely to occur under adverse climatic conditions in outdoor installations. This difficulty is attributable to the inherent characteristics of organic materials of this type which cause them to break down to form low-resistance carbonaceous deposits when exposed to creepage type electrical discharges. Composition 19 includes an organic material and hydrated alumina dispersed therein in sufficient quan- .tity to prevent accumulation of carbonaceous deposits upon exposure to creepage electrical discharges such as those which may occur between the conducting members 10,11, 12, 13 and 20.

With apparatus not embodying our teachings, it is believed that the breakdown process with respect to creepage failures occurs as follows. The localized high-tem- .perature discharges produced across the surface of the 'organic composition breaks down the organic components to form carbon deposits. The process is cumulative, as :has been previously desired, and low-resistance tracking failure occurs rapidly.

Electrical apparatus wherein exposure to contaminating conditions is likely to occur and in which our teachings may be embodied to great advantage also includes apparatus other than current transformers which are unprotected from such contaminating conditions. In accordance with one practice of our invention, we provide high-voltage electrical apparatus in which con- :ductorshaving a large potential difference are found with an insulating organic material having interspersed therein a hydrated alumina in proportions such that material breakdowns ordinarily experienced under creepage electrical discharge conditions do not occur. The concen- :tration of the hydrated alumina employed is critical since it has been found that only a sufiicient concentration Jelirninates carbon accumulation and electrical breakdown. 'Aswill be demonstrated, this effect can be achieved by employing a hydrated alumina only in the hereinafter defined critical proportions. A very marked increase in :operating life of the apparatus under creepage electrical discharge conditions is observed as the critical levels of concentration are reached.

.It has been found that when the hydrated alumina com- .prises 25 to 65% by weight of the combined Butyl gumhydrated alumina insulation, failure due to creepage is substantially eliminated. When the hydrated alumina constitutes fromabout 40 to 65% by'weight of the composition failure due to creepage trackage is wholly eliminateed. Using epoxy resin, the hydrated alumina effectively prevents creepage failure when present in the amount of 20 to 70% byweight of the insulation and preferably 40 to 70%. When polyesters are used, the hydrated alumina is used in the proportion of from 20 to 70% by Weight of the insulation and 30 to 70%. The preferred specific hydrated alumina content of each insulating composition is 60% by weight.

While we do not wish to be bound by any particular theory, it being sufiicient that our invention accomplishes the desired end, we believe that the combined water in the hydrated alumina serves to oxidize the carbonaceous .particles formed under creepage conditions and that the aluminum oxide component itself acts as a catalytic agent to indirectly promote this oxidation. It has been found that while unhydrated aluminum oxide is useful in delaying tracking under surface creepage conditions, it is not efiicacious in oxidizing carbonaceous materials or in preventing the eventual failure of insulation due to creepage tracking. It has also been found that the water must be chemically bound to the aluminum oxide as in the hydrated compound. It will be obvious that .inpractical commercial applications failuredue to surface creepage tracking must be not merely delayed but wholly eliminated as by the practice of our invention.

In order that those skilled in the art may better und'erstand how the present invention may be practiced, the following examples are given to illustrate organic insulation components containing organic materials coming within the scope of this invention. These examples are given by way of illustration and not by way of limitation.

EXAMPLE I Tests were conducted with apparatus mock-ups including various insulating compositions including Butyl rubber and selected concentrations by weight of a hydrated alumina of the chemical composition Al O .3H O. The test results are graphically presented in FIGURE 3 of the drawings, the actual test point averages for each concentration of hydrated alumina being indicated by the plotted points.

The precise numerical values of the test points graphi- The remaining material employed in each of the above listed elastomeric organic insulation components was, in each case, a curing system along with diatomaceous earth as a filler. The curing system consisted of the following materials in the percentages by weight of the amount of Butyl gum employed.

TablaII Percent Stearic acid -3 Zinc oxide (ZnO) r 5 .Red lead (Pb O4) 1 Elemental sulfur 2 p,p-Dibenzoylquinonedioxime V 6 The samples of insulation components used in these tests were prepared as molded sheets about .076" thick and about 6 square and were mold cured at C. for about 20 minutes. ,For testing purposes, these samples, identified by numeral .21 in FIGURE 4, were mounted in a'high voltage test cage resting flat against a conducting metal plate 22 tilted at an angle of about 15 from the horizontal.

Two electrodes, 23 and 24, each about v1" by 2" in cross section were placed perpendicularly against the surface of each sample 217under test, that is, against the upper side of the .sample opposite that in contact with the metal plate 22. The electrodes 23 and 24 were positioned about 1" apart with the 2 dimensions of the electrodes extending parallel to each other. One of the two electrodes, identified by numeral 23, was connected to the metal plate 22 by clips 25 and 26 and conductor 27. The two electrodes were electrically connected across the alternating voltage output of an adjustable high-voltage transformer through clips 28 and 29. It will be appreciated that, with the circuit just described, creepage breakdown of the, sample may occur between the two electrodes 23 and 24 or between electrode 24 and the metal plate 22 by means of a surface failure, or between electrode 24 and the metal plate 22 through the thickness of thesample by means of an erosion type failure.

After the test samples were placed in the apparatus as just described, they were dusted with a synthetic dust representative of atmospheric dust accumulations, the dust particles being designated by reference character 30. A fine water spray 31 was directed against the samples for the duration of the test, a nozzle 32 being coupled with water and air lines 33 and 34 for this purpose. A 60-cycle voltage of about 1500 volts was applied to the electrodes 23 and 24 to set up surface discharge conditions of the intensity capable of causing decomposition of organic materials. The samples were removed from the apparatus for redusting purposes at the end of each 100 hours of testing, or sooner for inspection purposes if failure occurred.

It will be observed by referring to the dust-spray test data presented in Table I and the graphical presentation thereof as shown in FIGURE 3, that a large increase in operating life of the apparatus under electrical discharge conditions is obtained as the concentration of hydrated alumina is increased beyond a certain level. A more significant consideration is, however, the concentration level at which the type of failure encountered changes from a surface" type failure to an erosion type failure.

The surface failure is one which occurs by reason of formation of a random carbon path along the surface of the material indicating that the life of the material is being limited by carbonization, while an erosion failure is one which occurs because the insulation component 21 is reduced in thickness by an eroding effect not involving carbonization. That is, it is determined by the inherent qualities of the insulation component itself rather than by some external cause such as carbonization.

Hence, at the point where erosion failure begins to occur instead of surface failure, the concentration of hydrated alumina has reached a level where carbonization is no longer the limiting factor with respect to ability of the high-voltage apparatus to withsand creepage elecrical discharges. The samples used to obtain the data presented in Table I were inspected upon failure to determine the type of failure involved in each case; that is, Whether breakdown occurred across the surface through carbonization or whether it occurred through the thickness thereof by erosion between the ungrounded electrode 24 and the metal plate 22. The types of failures encountered in each case are set forth in Table I.

At a level of about 25% hydrated alumina concentration by weight of the whole insulation component, both types of failures were encountered; that is, certain samples failed by reason of erosion while others failed by reason of surface tracking, some in the latter category showing serious erosion damage. At 30% hydrated alumina concentration, all failures were of the erosion type and average operating life before breakdown of the apparatus was noted to be 190 hours. The critical concentration of hydrated alumina for the particular Butyl rubber formulation employed, and under dust contamination conditions, would therefore appear to extend upward from 25% by weight of the whole, with the expected operating life under the test conditions set forth averaging about 200 hours as compared to a 2-hour average life under the same conditions with apparatus wherein the concentration of hydrated alumina in the organic insulation component was only about 17% by weight, a figure representative of prior art formulations.

Example I was repeated except that insulating compositions containing hydrated alumina on actual transformers such as in FIGURE 5 were exposed to a heavy salt fog containing by weight NaCl and 1.2% MgCl in a substantially closed space and subjected to a potential difference of 5 kv. between electrodes or conductive elements with a separation of 13 inches. The

results obtained during this test which represents the anost severe operating conditions were as follows:

Table III Percent Percent Hours to Hydrated Butyl Failure Type Failure Alumina Gum 17 40 Surface.

52 35 1, 300 No sign of Carbonizetlon.

The severity of the salt fog test described above is illustrated by the fact that the insulating composition tested thereunder containing 37% by weight hydrated alumina failed by surface erosion whereas under the dust-spray test of Table I an insulating composition containing only 30% by weight hydrated alumina resisted surface failure.

At or above the optimum concentration of hydrated alumina, the operating life of the apparatus is largely a function of the thickness of the insulation component, that is, the thickness employed between two conducting members of different potential. Below the optimum point, the operating lifeis still limited bycarbonization, and changes in thickness have a relatively minor, ifany, efiect on operating life. In general, the quantity ofhydrated alumina which can be employed with the organic Butyl insulation components in the practice of this invention ranges from about 25% to about 6.5% by weight of the whole, although a preferred range effective under the most deleterious creepage conditions extends from about 40% to about 65% by weightjof the whole. At concentrations above about 65% by weight of the hydrated alumina, there is a gradual loss of the desirable physical characteristics of the composition;

' t EXAMPLE II c si g co e p i to o mu ations vA an B p ctively of Table IV below. These transformers were all rated :5 kv. and an alternating 60-cycle volt-age of 5 kv. was applied to the transformers continuously throughout the test. I

' Table IV Butyl Gum a... Zinc Oxide (Z)v Stearlc a d Hydrated alumina (Al2O .3HiO) Processing 011 Parafl-ln wax Mineral rubber Elemental sulfur p,p'-Dibenzoylquinonedioxime Red lead (PbBO4) It will be observed that in formulation A, the amount of hydrated alumina is approximately 16.9% by weight of the whole, while in fonmulation B the amount of hydrated aluminav is about 59.3% by weight 'of the I I r The outdoor current tr ansformer shown in FIGURE 5 includes a magneticcore 36 about which are disposed high-voltage primary windings 37 and lower-voltage secondary windings 38. High-voltage conductors or terminals 39 project outwardly from the molded casing and insulating component 35 and serve -to couple the primary windings 37 in series with a high-voltage transmission line. Connections to the secondary windings 38 are completed through conduits 40 and terminals disposed behind the cover plate 41. A metal support plate 42 facilitates mounting of the apparatus. Despite the provision of anti-creepage ridges 43, surface creepage discharges may occur between the terminals and between these terminals and the conducting support plate 42, particularly where surface contamination is experienced.

Of the group of such transformers having insulation components of formulation A of Table IV, all had failed by carbonized tracking after 2400 hours of testing, whereas of the second group of transform rs, which included formulation B of Table IV, none showed any damage of any kind at the time of inspection after the test had progressed for over 21,000 hours.

The aforementioned group of transformers including the formulation B were of the solid type. For purposes of a further comparison, like transformers were assembled including an inner insulating component having the formulation A of Table IV and further having an outer jacket insulating component of thickness from to inch. This jacket fully surrounded and was molded to the inner composition, the jacket formulation being formulation C in'Table IV. These transformers have also exceeded 21,000 hours of the same testing without failure EXAMPLE [III In conectionwith other types of organic insulation components, dust-spray tests were conducted with an epoxy casting resin or ethoxyline resin) identified as Shell Chemical Company epoxy resin No. 828. The resin was mixed-in different batches with the percentages of hydrated alumina (Al O 3H O) by weight of the whole as indicated below in Table Vtogether with 8.5% by weight of diethylene triamine :as a catalyst based on the weight of the resin.

Test sheet samples of approximately .075" in thickness were cast and mold cured at 125 C. for about one hour. The tests were conducted in accordance with the procedure set forth in 'Eaxample *1 and in the apparatus of FIGURE 4, and the results tabulated below were obtained.

8 EXAMPLE V Example III was repeated using as the epoxy resin Araldite No. 6010 made by Ciba Company and as the curing agent 12.5% by weight of triethanolamine based on the weight of the resin. When tested as in Examples 1 and III the results were as follows:

Example III was repeated using pht'halic anhydride in the amount of 40% by weight of the resin as the curing agent for Shell Chemical Company epoxy resin No. 828. When these materials were tested as in Examples I and III, the results were asfollows:

Table VIII Material Percent Hours to Failure Type of A170 -3Hz0 Failure 0 Surface. 10 Do. 20 Do. Shell Chemical Co. 30 Surfaceand epoxy resin No. Erosion.

60 Greater than 200.... Erosion. 60 6. Do. 70 Do.

EXAMPLE VII Example III was repeated using as the curing agent for Ciba Company epoxy resin 6060, phthalic anhydride in the amount of 30% by weight of the resin. When tested as in Examples I and III, the results were as follows:

centration of Al O .3H O; that is, the concentration level Where carbonization is no longer the limiting factor on operating life, is also in excess of 25% by weight of the total insulation.

EXAMPLE IV Example III was-repeated-using-as the curing agent triethanolamine in the amount of 12.5% by Weight of the resin. The results of tests conducted as in Example III on this material are tabulated below:

Table V Table IX Materiel Percent iHours to Fallure Type of Material Percent Hours to Failure Type of Alz0a-3H1O Failure Alz0s-3Hz0 Failure 28 glsess than 1 gurtece. d 18 hess than 1 surfia ce. in ace an 0. Shell Chemical Erosion. Gib O 20 D0. epoxy resln No. 30 Greater than-200 "Erosion. e a z g ab 30 Do. 828. 40 ....do Do. gf g g r 40 Surface and 50 do Do. Erosion. -66 "do Do. 50 Greater than 200..-. Erosion; V .do Do.

It is noted that with this epoxy resin, the critical con- 55 It will be noted from the above examples directed to epoxy resins that in general the amine type curing agents produce insulating materials according to this invention which are more resistant at lower hydrated alumina concentrations to creepage failure than epoxy resins cured with acid anhydrides, it being necessary with the latter curing agent to add relatively more hydrated alumina to afford complete protection. It will be also noted that from about 20% to 70% of hydrated alumina based on the total weight of the material provides insulation which prevents creepage under the dust-spray test. In the salt fog test described hereinbefore about 40% by weight or EXAMPLE VIII Example I was repeated except that the resin used was a commercially available polyesterresin known as Lam- 9 inac No. 4116 manufactured by American Cyanarnid Company to which was added 1% by weight of tertiary Butyl perbenzoate, as a catalyst, based on the weight of the resin. When this material with added hydrated alumine was tested as in Examples I and III, the results were as follows:

Example I was repeated except that the polyester used was H. H. Robertson Companys Stypol 107E catalyzed with 0.5 .part of benzoyl peroxide per each 100 parts by weight of resin. When hydrated alumina was mixed with the resin in varying amounts and the material tested as inExamples I and III, the results were as follows:

Table XI Material Percent Hours to Failure Type of AlgOa-3H2O Failure 1 Surface. 10 50 SuFri'ac'e1 and H H. Robertson 20 180 Erosion. Btypo so Greater than 200 Do. 40 do Do. 50 -do Do.

From Examples VIII and IX it will be seen that polyesters mixed with hydrated alumina also exhibit desirable creepage resistance. Under dust-spray condition creep tracking is prevented when from 20 to 70% by weight of the composite insulation is hydrated alumina. Under salt fog conditions a minimum of about 30% hydrated alumina is preferred to eliminate creep tracking. Again, the upper limit is imposed only by the physical requirements of the insulation.

The term Butyl rubber or Butyl gum as employed herein is intended to mean broadly, a solid, rubbery copolymer or interpolymer comprising the product of polymerization of a mass of copolymerizable materials containing, by weight, a major proportion (e.g., from 60 to 99%) of an olefin (monoolefin), more particularly an isoolefin, e.g., isobutylene, 2-ethylbutene-1, etc., and a minor proportion (e.g., from 1 to 40%) of a conjugated diolefin, e.g., butadiene-1,3, isoprene, cyclopentadiene, pent-adiene- 1,3, hexadiene-2,4, etc. The Butyl gums noted in connection with the examples above were copolymers of about 98 parts isobutylene and 2 parts isoprene.

The term epoxy resin or ethoxyline resin as used herein is intended to mean a complex epoxide resin comprising a polyether derivative of a polyhydric organic compound, e.g., polyhydric alcohol or phenols containing at least two phenolic hydroxy groups, said derivative containing 1,2 epoxy groups. The ethoxyline resins as just defined are disclosed in various places in the art such as in Castan Patent 2,324,483 as well as Castan Patent 2,444,333, British Patent 518,057 and British Patent 579,- 698. US. Patents 2,494,295; 2,500,600; 2,511,913 and 2,691,007 disclose other examples of ethoxyline resinous compositions falling within the definition. Curing of such compositions is readily accomplished by acidic (e.g., phthalic anhydride, etc.) or strongly alkaline materials (e.g., diethylene triamine, etc.).

The term polyester resin refers to a material comprising the reaction product of a polyhydric saturated or unsaturated alcohol and a saturated or unsaturated polybasic acid either with or without a modifying unsaturated monomer such as styrene, etc. Specific examples of the basic material are, for instance, diethylene glycol maleate, dipropylene glycol maleate, diethylene glycol fumarate and so forth. Such materials are readily polymerized by peroxy catalysts such as benzoyl peroxide, tertiary Butyl perbenzoate, etc.

It will also be apparent that the particular examples set forth above by way of illustration are not limiting in nature and that various forms of electrical apparatus having greatly improved resistance to breakdowns caused by creepage electrical discharges may include insulation components having the organic insulating materials set forth above and a hydrated alumina in concentration defined by the foregoing disclosure, and that such arrangements deriving the advantages of our invention, will fall within the true scope and spirit of our invention as defined in the appended claims.

What we claim as new and desire to secure by Letters, Patent of the United States is:

1. Electrical apparatus comprising at least two spaced electrically conducting members between which electrical potentials may be developed, an organic insulating material disposed and completely filling the space between said members and having an outer surface intermediate said members exposed to ambient contaminating atmospheric conditions, said surface material comprising Butyl gum which tends to form low resistance carbonaceous deposits under the influence of creepage type electrical discharges occurring under contaminating conditions and means for preventing the formation of said carbonaceous deposits due to creepage, said means comprising hydrated alumina interspersed in said surface material in the amount of from 25 to 65 percent based on the weight of said surface material and hydrated alumina.

2. Molded electrical apparatus comprising at least two electrically conducting members between which electric potentials may be developed and an insulating composition disposed between said members and having an outer surface intermediate said members exposed to ambient contaminating atmospheric conditions and fixing said members in spaced relationship such that electrical discharges of the creepage type may occur therebetween and across said surface of said molded insulating composition,

said composition including Butyl gum which leaves a low resistance carbonaceous residue under influence of creepage type electrical discharges and further including hydrated alumina interspersed in said position in the amount .1 of from 25 to 65 percent based on the weight of said composition and hydrated alumina, said hydrated alumina preventing accumulation of carbonaceous material upon exposure of said surface of said insulating composition .1 and hydrated alumina to creepage type electrical dis-- charges occurring across said surface under contaminating conditions.

3. An electrical insulator shaped to support and holdf in insulating relationship a plurality of conducting members which are adapted to have electrical potentials de-- veloped thereacross and between which electrical dis-- charges of the creepage type may occur, said insulator comprising an organic insulating material disposed between said members and having an outer surface intermediate said members exposed to ambient contaminating atmospheric condi ions, said surface material comprising Butyl gum which tends to form low resistance carbonaceous deposits under influence of cree age type electrical discharges and hydrated alumina dispersed in said sur face material and comprising from about 25 to 65 percent by weight of the combined surface material and hydrated alumina, said hydrated alumina serving to prevent the formation of said carbonaceous deposits under said contaminating conditions.

4. Electrical apparatus comprising at least two spaced electrically conducting members between which electrical potentials may be developed, an organic insulating material disposed and completely filling the space between said members and having an outer surface intermediate said members exposed to ambient contaminating atmospheric conditions, said surface material comprising Butyl gum which tends to form low resistance carbonaceous deposits under the influence of creepage type electrical discharges occurring under contaminating conditions and means for preventing the formation of said carbonaceous deposits due to creepage, said means comprising hydrated alumina interspersed in said surface material in the amount of 60 percent based on the weight of said surface material and hydrated alumina.

5. Electrical apparatus comprising at least two spaced electrically conducting members between which electrical potentials may be developed, an organic insulating material disposed and completely filling the space between said members and having an outer surface intermediate said members exposed to ambient contaminating atmospheric conditions, said surface material comprising Butyl gum which tends to form low resistance carbonaceous deposits under the influence of creepage type electrical discharges occurring under contaminating conditions and means for preventing the formation of said carbonaceous deposits due to creepage, said means comprising hydrated alumina interspersed in said surface material in the amount of from 40 to 65 percent based on the weight of said surface material and hydrated alumina.

6. In a molded electrical transformer adapted to be;

energized by a source of electrical power and having spaced high voltage terminals and low voltage terminals, an insulating material molded about said transformerand' fixedly positioning said terminals, said material being dis posed and completely filling the space between said termi-' n'als and having an outer surface intermediate said high and low voltage terminals exposed to ambient contaminat-' ing atmospheric conditions, the improvement which comprises at least the outer surface of said material including butyl gum which leaves a conducting carbonaceous deposit under'the influence of creepage electrical discharges and hydrated alumina dispersed in at least said outer surface of said material to comprise from 25 to percent by weight of at least said outer surface of the combined insulating material and said hydrated alumina, said hydrated alumina obviating said carbonaceous deposits.

References Cited in the file of this patent UNITED STATES PATENTS 2,371,915 Rector Mar. 20-, 1945: 2,618,689 Cook Nov. 18, 1952; 2,645,626 Nordlander et a1 July 14, 1 953 2,679,493 Anderson May 25, 1954 2,768,264 Jones et al Oct. 23, 1956 v UNITED STATES PATENT. OFFICE CERTIFICATE or CORRECTION Patent No. 2,997,526 August 22,, 1961 Alvan A. Kesse-l et a1.

pears in the above numbered pat- It is hereby certified that error ap d Letters Patent should read as ent requiring correction and that the sai corrected below.

Column 1, column 3, lines 52 and 53,

line 46, for "ultimaely" read ultimately for "eliminateed" read eliminated column 4, line 21, for "table" read whole column 5 d electrical column 7, line line 39 for '"elecrical" rea 31, before "or" insert an opening parenthesis; column 10,, line 46, for "position" read composition Signed and sealed this 10th day of April 1962.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CURRECTION Patent No, 25997526 August 22 1961 Alvan A. Kessel et al.

It is hereby certified that error appears in the above numbered patid Letters Patent shouldread as ent requiring correction and that the sa corrected below.

Column 1, line 46,, for "ultimaely" read ultimately column 3, lines 52 and 53, for "eliminateed" read eliminated column 4, line 21, for "table" read whole column S line Z39 for "elecrical" read electrical column 7, line 31, before "or" insert an opening parenthesis; column 1O line 46, for "position" read composition --i Signed and sealed this lOth day of April 1962,

(SEAL) Attestz ERNEST W. SWIDER Attesting Officer DAVID L. LADDQ Commissioner of Patents 

1. ELECTRICAL APPARATUS COMPRISING AT LEAST TWO SPACED ELECTRICALLY CONDUCTING MEMBERS BETWEEN WHICH ELECTRICAL POTENTIALS MAY BE DEVELOPED, AN ORGANIC INSULATING MATERIAL DISPOSED AND COMPLETELY FILLING THE SPACE BETWEEN SAID MEMBERS AND HAVING AN OUTER SURFACE INTERMEDIATE SAID MEMBERS EXPOSED TO AMBIENT CONTAMINATING ATMOSPHERIC CONDITIONS, SAID SURFACE MATERIAL COMPRISING BUTYL GUM WHICH TENDS TO FORM LOW RESISTANCE CARBONACEOUS DEPOSITS UNDER THE INFLUENCE OF CREEPAGE TYPE ELECTRICAL 